The Basics of Probe Measurements on Wafers and Other Substrates

High Frequency Design
PROBE MEASUREMENTS
From May 2011 High Frequency Electronics
Copyright © 2011 Summit Technical Media, LLC
<strong>The</strong> <strong>Basics</strong> <strong>of</strong> <strong>Probe</strong>
<strong>Measurements</strong> on Wafers
and Other Substrates
By Gary Breed
Editorial Director
Microwave measurements
<strong>of</strong>ten
Here is an introduction to
the applications and technical
issues for microwave probe (Fig. 1) be used to
require that a
probe measurements <strong>of</strong> contact the circuit conductors
on the substrate
various circuit types
upon which the circuit is
constructed: typically a semiconductor wafer,
ceramic substrate or microwave laminate
material. This tutorial reviews some <strong>of</strong> the
applications for these measurements, and presents
the key issues affecting measurement
accuracy and repeatability.
Figure 1 · <strong>Probe</strong>s for microwave device
measurements, mounted in adjustable
probe holders [1].
Common <strong>Probe</strong> <strong>Measurements</strong>
On-wafer testing—Production testing <strong>of</strong>
semiconductor die is <strong>of</strong>ten performed after dicing
and packaging, but a sample must be tested
for two main reasons: verification that fabrication
was completed satisfactorily, and for
characterization <strong>of</strong> the bare die before packaging.
Device characterization at the die level is
essential, since packaging necessarily affects
performance. Knowing the device performance
before packaging allows the test engineer to
determine how various packaging options
affect device performance.
Developmental on-wafer circuits—It is common
practice to use sections <strong>of</strong> production
wafers for fabrication <strong>of</strong> test die for devices in
development. <strong>The</strong> ability to do on-wafer testing
eliminates the need for packaging the devices,
or mounting them to some other test fixture.
Characterization <strong>of</strong> passive components—
Individual circuit elements are combined for
any practical RF/microwave/high speed function.
With the present strong reliance on modeling
and simulation prior to actual fabrication
and prototyping, accurate characterization
<strong>of</strong> capacitors, inductors, resistors and
transmission lines are needed to develop the
necessary mathematical models for inclusion
in the s<strong>of</strong>tware tools. <strong>The</strong> components may be
segments <strong>of</strong> a monolithic circuit or separate
discrete devices.
Package characterization—Not can individual
devices be characterized, but packages
can be tested to determine their specific performance-modifying
effects. Figure 2 shows
two types <strong>of</strong> packages that might be used with
single transistors or multi-function ICs.
Non-monolithic integrated circuits—
Whether integrated on ceramic, microwave
laminate, or other substrate, microwave circuits
require testing. Today, very small-size
assemblies are common, with miniature discrete
components and thick film fabrication
78 High Frequency Electronics

High Frequency Design
PROBE MEASUREMENTS
techniques. <strong>The</strong>se circuits are <strong>of</strong>ten
tested using probing techniques
rather than the connectorized fixtures
that were used in the past.
Any <strong>of</strong> the above measurements
may be use for either R&D or quantity
production.
Key Issues for <strong>Probe</strong>
<strong>Measurements</strong>
1. <strong>Probe</strong> performance
Whether the chosen probe is coaxial
or coplanar in structure, the following
issues must be considered.
Contact pressure—<strong>The</strong> probe
must have sufficient force to make
reliable electrical contact, but must
not deform either the probe structure
or the DUT contact point. Commercial
probes are built with these factor
in mind, and the manufacturers provide
data on setup and maintenance.
Repeatability—<strong>The</strong> probe characteristics
should remain constant over
a significant number <strong>of</strong> placements
and measurements.
RF performance—<strong>The</strong> probe must
have known RF characteristics, with
constant impedance (minimal discontinuities)
from the probe tip to the
instrumentation connector.
Calibration procedures—This is
the most-referenced issue with probe
measurements. Like all microwave
measurements, instrument interconnections
can have a large effect.
Standard TRL (Termination, Reflection
and thru-Line) calibration methods
are more difficult to make at the
probe tip than with connectors. <strong>The</strong>
choice <strong>of</strong> termination, short circuit
and line section must match the
intended use <strong>of</strong> the probe. See
References [3, 4] to begin your indepth
study <strong>of</strong> probe calibration.
2. Fixture performance
Reference plane de-embedding—
With the DUT mounted on a carrier
substrate or clamped fixture, the calibration
<strong>of</strong> the probe must be extended
to the actual DUT connection. This
may be done with TRL calibration,
but matching DUT connection points
Figure 2 · Two types <strong>of</strong> packages that can be characterized using
microwave probe testing [1].
may be difficult. Calibration may be
simplest using probes to measure the
characteristics <strong>of</strong> the segment from
the probe contact to the DUT placement
point.
Parasitics—Any fixture must
have the best possible impedance
match, low inductance and low stray
capacitance. A fixture should be
designed for low radiation loss as
well, especially at the higher
microwave frequencies.
Device mounting—<strong>The</strong> method
placing the DUT into the fixture by
clamping or soldering should not
modify the electrical characteristics.
Otherwise, the measurements cannot
be trusted. This is a minor issue with
a well-designed commercial fixture,
but must be addressed thoughtfully
with in-house fabricated fixtures and
adapters.
3. System factors
<strong>Probe</strong> mounting—<strong>Probe</strong>s must be
securely fixed in position. <strong>The</strong> probe
station or probe card must also have
flexible mounting options, to accommodate
the range <strong>of</strong> devices to be
tested.
Connection to instruments—High
quality connectors and support for
interconnecting cables, with minimum
bending, are essential for any
probe measurement system. This is
good practice for all microwave measurements,
not just with probes.
DUT mounting—As noted above,
the DUT must be reliably mounted.
In automated production test systems,
accurate positioning is essential
since there are multiple connections
that must be maintained during
the series <strong>of</strong> tests.
Summary
<strong>Probe</strong> testing <strong>of</strong> microwave
devices and circuits is a big part <strong>of</strong>
both development and production. In
development, it is the means for
obtaining good models for simulation,
and for verifying performance <strong>of</strong> prototypes
and pre-production samples.
In production, accurate probe measurements
are needed to verify process
yield and consistent performance
from one batch to the next.
Accuracy, reliability and repeatability
require knowledge and attention
to the performance factors described
in this note.
References
1. “Precise, Repeatable RF <strong>Measurements</strong>:
Applying CPW <strong>Probe</strong>s to
Everyday Test Problems,” available
at www.jmicrotechnology.com
2. E. Strid, R. Gleason, K. Jones,
“40 GHz On-Wafer <strong>Measurements</strong>
with the HP 8510 Network Analyzer
and Cascade Microtech Wafer
<strong>Probe</strong>s,” www.hparchive.com/seminar_notes/a-133.pdf
3. D. F. Williams and R. B. Marks,
“Calibrating On-Wafer <strong>Probe</strong>s to the
<strong>Probe</strong> Tips,” at www.nist.gov
4. Extensive references on probe
calibration and measurements are
available at: www.nist.gov/pml/electromagnetics/rf_electronics/
80 High Frequency Electronics